Abbott sickened by journalist beheading

Prime Minister Tony Abbott says he's sickened by news that Islamic State extremists have beheaded an American journalist.

The group has released a video showing a masked militant executing James Foley, who has been missing since he was seized by armed men in Syria in November 2012.

Mr Abbott says he hasn't seen the video but is appalled by the reports.

"It's absolutely sickening, absolutely despicable," he told Brisbane's 4BC radio.

"It's a sign that there can be no compromise whatsoever with the murderous terrorists of the ISIL movement. This is as near to pure evil as we are ever likely to see."

Foley was an experienced freelancer who had covered the war in Libya before heading to Syria to follow the revolt against Bashar al-Assad's regime for the Global Post and AFP.

In the five-minute video, distributed online by known Islamic State sources, the group also known as ISIL or ISIS declares Foley was killed after US President Barack Obama ordered air strikes against the IS in northern Iraq.